Theme A: The role of releases of cultured animals in fisheries management and ecosystem restoration: integrative evaluationKEYNOTE: Evaluating releases
7&8. QUANTITATIVE APPROACHES TO EVALUATING THE CONTRIBUTION OF RELEASE PROGRAMS TO FISHERIES MANAGEMENT GOALS
Professor Kai Lorenzen
Program in Fisheries and Aquatic Sciences
School of Forest Resources and Conservation
University of Florida
7922 NW 71st Street
Gainesville, FL 32653, USA
klorenzen@ufl.edu
Quantitative assessment of the contribution a release program can make to fisheries management goals, including synergies and tradeoffs with fishing regulations and habitat management, is a key requirement if enhancements are to be effective and sustainable. Population dynamics theory and quantitative assessment methods for enhanced fisheries have developed rapidly over the past decade. I provide a critical review of recent developments in the areas of population dynamics theory, the dynamics of alternative enhancement systems, assessment approaches, monitoring and experimental design, and reference points and management control rules. I close by outlining a set of best practice guidelines for quantitative assessments and priorities for further research.
Population dynamics models commonly used in fisheries assessment have been extended in various ways to allow evaluation of release programs. This includes ‘unpacking’ of the stock-recruitment relationship to describe dynamics in the pre-recruit stage explicitly; quantifying compensatory density-dependent processes in the recruited phase of the life cycle; accounting for differences in fitness between hatchery-released and wild fish; and explicitly modelling spatial dynamics. In several areas, such as the consideration of size-dependence in lifetime mortality schedules, models originally developed for enhanced fisheries have become widely used in the assessment of wild stocks.
Release programs can be used in different situations and for different purposes, which in turn give rise to very different approaches to population assessment and management. Five main types of marine fisheries enhancement systems may be distinguished, in a sequence ranging from the most production-oriented to the most conservation-oriented type: sea ranching, stock enhancement, restocking, supplementation and re-introduction. Ranching systems operate for species that do not recruit naturally and may be managed to maximize somatic production (commercial fisheries) or the abundance of catchable-sized fish (recreational fisheries), often manipulating population in ways that could not be achieved in naturally recruiting populations. Because direct genetic interactions with wild stocks are absent, post-release fitness of cultured fish is primarily an economic rather than a conservation issue. Stock enhancement involves the continued release of hatchery fish into a self-recruiting wild population, with the aim of sustaining and improving fisheries in the face of intensive exploitation and/or habitat degradation. Enhancement through release of recruits or advanced juveniles may increase total yield and stock abundance, but is likely to reduce abundance of the naturally recruited stock component through compensatory responses or overfishing. Stocking and harvesting rates in such fisheries are strongly constrained by stock conservation considerations. Impacts on the wild population component can be reduced by separating the cultured/stocked and wild population components as far as possible. Re-stocking involves time-limited releases of hatchery fish, aimed at rebuilding depleted populations more quickly than would be achieved by natural recovery. In re-stocking, release number must be substantial relative to the abundance of the remaining wild stock if rebuilding is to be significantly accelerated. Restocking cannot substitute for effort limitation, and is advantageous as an auxiliary measure only if the population has been reduced to a very low proportion of its unexploited biomass.
Quantitative assessments of fishery management contributions should be carried out at all stages of development of a release program, from early planning to full-scale operation. Model components and parameters may be estimated from three principal sources: (1) quantitative assessments of the wild stock, (2) release experiments with marked fish, and (3) comparative empirical studies and meta-analyses. Assessment of fisheries enhanced through hatchery releases requires more extensive monitoring than that of fisheries sustained by natural recruitment alone. In particular, wild and hatchery-origin fish must be distinguished and the fitness of hatchery fish and their hybrids with wild fish evaluated. Wherever possible, enhancements should be designed as experiments with spatial and temporal controls.
Reference points define targets or limits of stock status in fisheries management. Where release programs are carried out at operational scales, reference points should be defined for the combined stock and for its wild component. Reference points and management control rules for enhanced fisheries have received insufficient attention in research and fisheries governance.
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